As partly discussed in the authoritative book Holma, Toskala: WCDMA for UMTS, Radio Access for Third Generation Mobile Communications, John Wiley & Sons, Ltd, 2000, CDMA systems are interference-limited from both the receiver performance and system capacity points of view. The interference-limited nature of CDMA systems results from the receiver design: reception is typically based on a spreading code matched filter (MF) or a correlator. Since the received spreading codes are usually not completely orthogonal, multiple access interference (MAI) is present in the receiver.
Sub-optimal multi-user receivers have been developed for fighting multiple access interference. Multi-user receivers can be categorised in several ways: one is to classify the receivers to two main classes: linear equaliser and subtractive interference cancellation (IC) receivers. Linear equalisers, for example zero-forcing (ZF), de-correlating detectors or minimum mean square error (MMSE) detectors, are linear filters suppressing multiple access interference. The principle of an IC receiver is that the multiple access interference component is estimated, and then subtracted from the received signal for making decisions more reliable. Multiple access interference cancellation can be carried out in parallel for all users by using parallel interference cancellation (PIC) receivers, or in a serial fashion, by using serial interference cancellation (SIC) receivers.
In PIC receivers, detection is based on using tentative data decisions; de-spread bits are used to improve the performance of reception. The quality of tentative decisions is essential, because if a wrong tentative decision is made, interference in a received signal increases instead of decreasing.
The problem is that the performance of this kind of receivers is not optimal if repetition coding is used in a transmitter because the error rate of de-spread bits increases due to a lower effective coding rate (ECR).
There are prior art methods for increasing the reliability of tentative decisions. In one method, the received information block is decoded and decoded bits are used in estimating interference. The problem is that the decoding process is time-consuming and therefore causes delay.
In another solution re-transmitted data-blocks are utilised in making tentative decisions (automatic repeat request (ARQ) and hybrid automatic repeat request (HARQ)). Again, the problem is time delay: re-transmission is also a time-consuming process.